Biomedical Engineering Reference
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in the measurements are contributed by numerous molecules. These experimental
methods are called ensemble measurements in general. It would be very difficult, if
not impossible, to directly measure the experimental result of single molecules by
the ensemble measurements. To solve this problem, single-molecule experiments
are needed, which only manipulate a single molecule at a time. Here, we will
introduce several typical experimental methods for single-molecule mechanics and
the research progresses in the DNA field.
6.1
Experimental Methods for Single-Molecule Mechanics
The experimental methods for single-molecule mechanics have been developed
rapidly into power tools in the past 20 years. By utilizing magnetic tweezers,
optical tweezers, and atomic force microscope (AFM)-based force spectroscopy,
scientists have successfully characterized the intra- and intermolecular interactions
of many systems at the single-molecule level [
1
]. Single-molecule experimental
results have been directly measured, which are not accessible by the traditional
methods. Undoubtedly, the single-molecule studies have greatly extended the scope
of science and technology.
6.1.1
Magnetic Tweezers
Magnetic tweezers manipulate the individual molecule that linked to the superpara-
magnetic micro bead by controlling the position of the bead via the magnetic field
[
2
,
3
](seeFig.
6.1
). Therefore, this method is also called magnetic bead technique.
The typical size of the magnetic bead is from several nanometers to several microns.
When the external magnetic field is constant, the magnetic force applied on the
bead is increased with increasing size of the superparamagnetic bead. Similarly, for
a bead with given size, the magnetic force applied on the bead is increased with
N
S
objective
lens
Fig. 6.1
The scheme of
magnetic tweezers
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